Tires are typically prepared with treads comprised of diene-based rubber compositions which may contain particulate synthetic amorphous precipitated silica reinforcement, usually referred to as precipitated silica, together with a silica coupling agent having a moiety reactive with hydroxyl groups (e.g. silanol groups) contained on the surface of the silica and another, different, moiety interactive with diene-based elastomers contained in the rubber composition.
A precipitated silica reinforced rubber composition may be provided, for example, by the addition of a precipitated silica to a rubber composition containing a diene based elastomer together with a silica coupling agent to chemically react with the precipitated silica in situ within the rubber composition to couple the precipitated silica to the diene based elastomer of the rubber composition.
Alternatively, a precipitated silica reinforced rubber composition may be provided, for example, by the addition of a pre-hydrophobated precipitated silica, comprised of a precipitated silica pre-treated with a silica coupling agent, to a rubber composition containing a diene based elastomer to couple the precipitated silica to the diene based elastomer of the rubber composition.
Reinforcing fillers for diene elastomer based rubber compositions, such as for example precipitated silica and rubber reinforcing carbon black, tend to assume a preferential affinity for, and therefore a preferential reinforcement of, diene based elastomer(s) with which they are first blended. Therefore, it is considered that the order of addition of individual elastomers and precipitated silica to prepare a rubber composition can vary the effective reinforcement of the rubber composition which can, in turn, lead to rubber compositions with a variety of physical properties.
The methodology of this invention is considered to be a significant a departure from and exclusive of simply blending elastomer(s) and combination of precipitated silica (with a silica coupler) and pre-hydrophobated precipitated silica or sequentially blending a combination of precipitated silica (with its silica coupler) and pre-hydrophobated precipitated silica with elastomers.
This methodology is intended to be differentiated as a departure from a sequential order of mixing two precipitated silicas with a rubber composition where a first pre-treated (pre-hydrophobated) precipitated silica is first blended with a rubber composition followed by the addition of a second, non-pre-treated (non-pre-hydrophobated) precipitated silica, or vice-versa, or both of the precipitated silicas blended together with the elastomer in a rubber mixer (e.g. an internal rubber mixer such as, for example, a Banbury™ mixer). In such order of mixing, the first blended precipitated silica (e.g. as a pre-hydrophobated precipitated silica or the non-pre-hydrophobated precipitated silica) is considered to be primarily associated with the rubber composition to provide an attendant primary reinforcement contribution of the rubber composition as compared to the second blended precipitated silica. It is envisioned that various physical properties of the resultant rubber composition would therefore depend upon the order of sequential addition of the precipitated silicas.
For example, it is readily seen that, for such sequential mixing, a precipitated silica which has been pre-treated with a silica coupling agent may be reacted with diene based elastomer(s) which has already been reacted with a combination of precipitated silica and silica coupling agent, or vice versa.
Contrary to such sequential mixing, it is desired to evaluate the separate and individual mixing of pre-treated (pre-hydrophobated) precipitated silica with diene based elastomer(s) and the separate and individual mixing of precipitated silica and coupling agent with diene based elastomer(s) as individual and separate masterbatches thereof followed by blending such masterbatches together with sulfur vulcanization ingredients. In such manner it is envisioned that each of the pre-treated (pre-hydrophobated) precipitated silica and precipitated silica with its silica coupling agent can achieve individually separate affiliations with, and thereby separate reinforcements of, diene based elastomers for the ultimately mixed rubber composition masterbatches without significant interference from each other which may be expected from the aforesaid sequential mixing of the precipitated silica (together with silica coupling agent) and pre-treated (with silica coupling agent) with the rubber composition.
Therefore, the term “masterbatch” or “master batch” as used herein relates to blending the ingredients of the uncured rubber composition in what is usually referred to as “non-productive mixing”. For this invention, separately prepared masterbatches are blended together to form a blended rubber composition either together with sulfur curatives or prior to subsequent and separate mixing of sulfur curatives (e.g. sulfur and at least one sulfur vulcanization accelerator) with the blended rubber composition.
It is believed that such masterbatch mixing methodology for the aforesaid combined precipitated silica reinforcement of rubber compositions containing diene based elastomers is a significant departure from past practice.
In the description of the invention, the term “silica-rich” refers to a rubber composition in which, insofar as rubber reinforcing carbon black and precipitated silica reinforcing fillers are concerned, the reinforcing filler for the rubber composition is primarily precipitated silica.
The term “phr” where used herein, and according to conventional practice, refers to “parts of a respective material per 100 parts by weight of rubber, or elastomer”.
The terms “rubber” and “elastomer” where used herein, are to be used interchangeably, unless otherwise prescribed. The terms “rubber composition”, “compounded rubber” and “rubber compound”, if used herein, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients and materials” and such terms are well known to those having skill in the rubber mixing or rubber compounding art. The terms “vulcanize” and “cure” where used therein are used interchangeably unless otherwise indicated.